Black Beetle: Lessons from the Past and Options for the Future

119 Black beetle: lessons from the past and options for the future N.L. BELL1, R.J. TOWNSEND2, A.J. POpaY1, C.F. MERCER3, and T.A. JACKSON2 1AgResearch, Ruakura Research Centre, Private Bag 3123, Hamilton 2AgResearch, Canterbury Science Centre, Lincoln 3AgResearch, Private Bag 11008, Palmerston North [email protected] Abstract North Island, with an earlier report of an occurrence An outbreak of the pasture insect pest black beetle in Canterbury (Brown 1964) presumably a mistaken began in the Waikato and Bay of Plenty in 2007/8 and identity. Beetles were first reported from areas around has persisted. The extent and severity of damage caused Auckland in the late 1930s and by the late 1950s by black beetle during the current outbreak has focused had been observed as far north as Dargaville with its farmer and researcher attention on methods to maintain southern continuous extent being from the base of persistent pasture now and in future outbreaks. This the Awhitu Peninsula to the base of the Coromandel paper reviews previous research in combination with Peninsula with pockets of infestation reported around data from the current outbreak and relates these to the Gisborne area (Todd 1959). By the early 1970s the current pasture management practice. The possibility continuous distribution extended to the northern-most of being able to predict the distribution, occurrence tip of the North Island and southwards to Raglan on the and duration of black beetle outbreaks is explored west coast and across to Tauranga in the east, including while actual and potential means of controlling black a coastal strip to near the tip of East Cape (Esson 1973). beetle are outlined. We conclude that there are methods The occurrence in the Gisborne area was, by then, available to successfully renew pastures in the presence confirmed (Esson 1973). Watson (1979) considered of black beetle but that outbreak situations increase the distribution of black beetle to be confined to areas risks and may limit subsequent pasture persistence. with a mean annual soil surface temperature of 12.8°C There are fewer readily available options to maintain an which, based on current National Institute of Water and existing pasture and more research is urgently needed Atmospheric Research Virtual Climate Station (VCS) to provide these options. data (Tait & Woods 2007), would include a continuous Keywords: Heteronychus arator, paspalum, pasture, coastal strip around East Cape and down to Cape pasture renewal, pest resistance, pest tolerance, ryegrass Kidnappers in the east with a southerly distribution including a strip of the west coast to about Whanganui Introduction (Fig. 1). This indicates that the range has gradually Black beetle (Heteronychus arator (Fabricius)) was extended south over the last 30–40 years, incorporating first observed in New Zealand in the late 1930s (Todd southern parts of Waikato and Bay of Plenty and parts 1959) and has since become an established pest in of inland Hawkes Bay. This places new areas at risk of the northern part of the country. This review aims to significant damage during outbreaks. give background to black beetle research that has been conducted in New Zealand, to identify gaps in Lifecycle and flight behaviour knowledge which would assist in management of The black beetle has a single generation per year (Fig. this pest and to give some indications of the control 2). In warmer areas of the country, spring development methods currently available to land managers in new may be much quicker than shown in Fig. 2 with first- and existing pastures. stage larvae being observed in early October in sandy soil near Dargaville (Esson 1973). Feeding on grass Distribution – worldwide and entry to New Zealand roots close to the soil surface by the third-stage larvae Black beetle was originally described from the Cape from mid-January to March causes severe damage at of Good Hope, South Africa as Scarabaeus arator a time when plants are already under moisture and (Fabricius 1775). It is now known to be distributed temperature stress and inevitably kills plants. throughout southern and eastern Africa (Ethiopia Adult beetles can fly and large massed flights can be to South Africa) and has spread to South America, observed in the autumn of outbreak years, with Watson Australia (New South Wales, Queensland, South (1979) trapping >85,000 beetles in massed flights in Australia, Western Australia and Norfolk Island) and March 1975. Much smaller flights normally occur in New Zealand (CAB International 2000). spring from late September to December, but these In New Zealand, black beetle is confined to the upper are also massed such that most flights occur on few 120 Pasture Persistence – Grassland Research and Practice Series 15: 119-124 (2011) (≤four) evenings per season (Watson 1979). During cm square spade divots approx. 8-cm deep, across each outbreak years, spring flights may be considerably paddock. If the average number of beetles found in the larger than normal (P.D. King quoted in Watson 1979). paddock was above 10/m2 the pasture was considered to Initial autumn flights appear to be triggered by the first be at risk of suffering significant damage the following significant rainfall after adults emerge from pupae, summer. A damaging larval population was defined as with these and subsequent flights occurring when 40–60 larvae/ m2 dependent on a range of abiotic and soil surface temperatures exceed about 17°C at dusk biotic factors such as soil moisture, soil temperature with calm wind conditions (Watson 1979). Density- and host availability (King 1979; King et al. 1982). dependent spring flight migrations prior to oviposition Using pitfall traps to estimate black beetle populations may also occur (King et al. 1981d). Dispersal of the in spring pasture was an unreliable method of assessing beetles during autumn and spring potentially plays a beetle density (King et al. 1980). crucial role in infesting new pastures during outbreak years. Further research is needed to increase our Population levels and modelling understanding of this, particularly as it relates to the use Populations of black beetle undergo periodic outbreaks of endophyte and insecticidal seed coatings to reduce during which severe pasture damage occurs. Population black beetle infestations. modelling work conducted in the early 1980s (King et al. 1981d) attempted to understand what drove Hosts outbreaks and thereby allow prediction of them. That Grasses, including paspalum (Paspalum dilatatum) and work was largely carried out in paspalum-dominated ryegrasses (Lolium spp.), are the preferred hosts of both pastures which were common at that time and produced larval and adult black beetle (King 1976; King et al. more stable beetle populations than the ryegrass 1981c; King et al. 1981e; King et al. 1981f; King et pastures with which they were compared (King et al. al. 1981g; Todd 1959) and these also act as preferred 1981f). The ryegrass cultivars used (e.g., Grasslands oviposition sites (King et al. 1981b). Although black Ruanui (King et al. 1981e)) would most likely have beetle larvae will consume white clover (Trifolium contained standard endophyte (see Endophyte section repens) when given no choice (King et al. 1981g), it for explanation of endophyte strains). Therefore, the appears that legumes are generally unfavourable hosts decline in beetle populations they observed from (King et al. 1981a; Sutherland & Greenfield 1978). February to September in ryegrass plots, leading to the Carrots are suitable food for larvae and adults in the conclusion that ryegrass pastures were suitable habitats laboratory (King, 1981c; pers obs). It appears that soil only over the spring and summer periods (King et al. organic matter may act as a feeding stimulant for larval 1981f), may well have been an effect of the endophyte black beetle (King 1977), which may have implications status of the plants rather than their suitability as a food for the amount of plant damage incurred in peat soils. source per se. By sowing a non-host crop in spring such as From the models developed in the 1980s in brassicas, legumes or chicory it may be possible to paspalum, the critical factors to black beetle population disrupt larval feeding over summer thus breaking the increase were: thermal units above 15°C between 1 black beetle lifecycle and helping to ensure a low September and 30 November; and density-dependent population density is present once pastures are resown variation in natality (King et al. 1981d). The thermal in autumn. A cropping phase would also allow for units calculation for September–November seems to fit control of weedy host grasses such as paspalum and both paspalum and ryegrass data (King et al. 1981d) annual poa (Poa annua). but some of the other key factors in the beetle life cycle were thought to be more important beneath ryegrass Sampling methods for on-farm monitoring than paspalum. High soil moistures, for instance, appear To make timely treatment decisions or to adjust farm to be unfavourable for young larvae (King 1979). King management strategies which compensate for pasture et al. (1981d) noted that their model was preliminary production losses, farmers need a way to predict and needed to be tested with data added from a broader potential future black beetle populations. Watson et range of pasture types. Additionally they noted that adult al. (1980a) showed that it is possible to determine flight dispersal needed to be investigated and the impact whether individual paddocks were at risk of developing this may have on population dynamics incorporated damaging summer populations of black beetle larvae into the model. It is unclear, therefore, how well the by monitoring black beetle adults during the preceding population models developed previously fit with the spring. The monitoring system advocated required current situation and across a wider geographical range. approximately half a person-hour per paddock to assess Although climatic factors are obviously an important beetle numbers from nine representative samples, 20- determinant in black beetle population changes, the Black beetle: lessons from the past and options for the..

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